BIP91 and BIP148 are two different plans to implement Segwit in BTC. Beginning July 21 Signalling for BIP91 begins, and could lock in as soon as July 23, and activate as soon as July 25th!
Once activated, any blocks that aren't signalling readiness for Segwit will be rejected! So make sure you are mining on a segwit ready pool by this time.
If BIP91 isn't locked in by July 29th, we risk a chain split at which time you'll have to decide which chain to support (segwit or nonsegwit). Furthermore BIP148 will be activated on August 1.
If lock in fails for BIP148, again we are at risk of a split.
While their's more dates to consider, these are the important ones in the near term!